This invention relates to a novel smectic liquid crystal compound suitable as a component of ferroelectric liquid crystal materials.
At present, the TN (Twisted Nematic) display mode has been most broadly employed for liquid crystal display elements. This TN display mode has a number of advantages such as low driving voltage, small power consumption, etc., but it is still inferior in the aspect of the response rate to emissive display elements such as cathode ray tubes, electroluminescence displays, plasma displays, etc. A novel TN display element having a twist angle elevated to 180.degree. to 270.degree. has also been developed, but its response rate is still inferior. Efforts for various improvements have been made as described above, but a TN mode display element having a high response rate has not yet been realized. However, in the case of a novel display mode using a ferroelectric liquid crystal the research of which mode has recently been extensively made, there is a possibility of notable improvement in the response rate (see Clark et al, Applied Phys. Lett., 36, 899 (1980)). This mode utilizes chiral smectic phases exhibiting ferroelectric properties such as chiral smectic C phase (hereinafter abbreviated to SC*). It has been known that phases exhibiting ferroelectric properties are not only limited to SC* phase, but also include phases of chiral smectic F, G, H, I, etc.
Various specific features have been required for ferroelectric liquid crystal materials used for ferroelectric liquid crystal display elements practically used, but at present, there is no single compound which satisfies such requirements; hence it is necessary to use ferroelectric liquid crystal compositions obtained by blending some liquid crystal compounds and/or non-liquid-crystalline compounds.
Further, not only have ferroelectric liquid crystal compositions composed only of ferroelectric liquid crystal compounds been reported, but also it has been reported in Japanese patent application laid-open No. Sho 61-195187/1986 that when one or more kinds of compounds exhibiting ferroelectric liquid crystal phases are blended with compounds or compositions exhibiting achiral smectic C, F, G, H, I phase or the like (hereinafter abbreviated to SC phase or the like) as base substances, the resulting blend can constitute a ferroelectric liquid crystal composition as a whole.
Further, it has also been reported that using a compound or a composition containing the compound each exhibiting Sc phase or the like as a base substance, at least one compound which is optically active but exhibits no ferroelectric liquid crystal phase is mixed with the above compound or composition to constitute a ferroelectric liquid crystal composition as a whole (see Mol. Cryst. Liq. Cryst. 89, 327 (1982)).
In a brief summary of these facts, it is seen that when one or more kinds of optically active compounds are blended with base substances, the resulting blend can constitute a ferroelectric liquid crystal composition irrespective of whether or not the optically active compounds exhibit ferroelectric liquid crystal phases.
As such base substances, various compounds exhibiting achiral smectic liquid crystal phases such as the Sc phase or the like may be used, but practically, liquid crystal compounds exhibiting smectic C phases within a broad temperature range including room temperature or mixtures thereof are preferred. Examples of components of these smectic C liquid crystal mixtures are liquid crystal compounds of phenylbenzoates, Schiff's bases, biphenyls, phenylpyridines, 5-alkyl-2-(4-alkoxyphenyl)pyrimidines, etc.
For example, Japanese patent application laid-open No. Sho 61-291679/1986 and WO 86/06401 disclose that mixtures of 5-alkyl-2-(4-alkoxyphenyl)pyrimidine compounds having Sc phase with optically active compounds are used as a ferroelectric liquid crystal material.